In recent years, according to the improvement of the performance of physical machines, researches of a virtualization technique for integrating a plurality of virtual machines (VMs) in one physical machine are ongoing. In the virtualization technique, for example, virtualization software (hypervisor) allocates the physical machine to the plurality of virtual machines and enables provision of services by application programs installed in the virtual machines. In recent years, a provider that provides a use environment of the virtual machines such as resources and infrastructures of the physical machine (hereinafter referred to as infrastructure provider) rents out the virtual machines to a provider that provides, for example, services using the virtual machines (hereinafter referred to as service provider). The infrastructure provider rents out the virtual machines to the service provider on the basis of conditions prescribed by a contract.
The virtual machines sometimes have a function of storing a state of a system at a certain point in time (hereinafter referred to as snapshot function). With this function, for example, when a failure occurs, the service provider is capable of taking measures for, for example, restoring the system to a state before the occurrence of the failure. On the other hand, for example, when a user who uses a service by the service provider performs an unauthorized access or the like, the snapshot function enables the service provider to erase a use history or the like of the user. Therefore, the service provider disposes, for example, in the physical machine in which the virtual machines are created, software for monitoring the virtual machine (hereinafter referred to as monitoring software) or the like and monitors changes of a state including restoration of the virtual machines (see, for example, Japanese Laid-open Patent Publication No. H8-278967, Japanese Laid-open Patent Publication No. 2011-186783, Japanese Laid-open Patent Publication No. 2007-207271, and Japanese National Publication of International Patent Application No. 2002-544636).